The present invention relates to colour display tubes having a screen with a two-colour penetron phosphor which luminesces in, for example, the primary colours of red and green, and another phosphor luminescing in a third primary colour such as blue.
Penetron screens are known and are discussed in an article "Performance of Penetration Color CRTs in Single-Anode and Dual-Anode Configurations" by G. R. Spencer in Proceedings of the SID, Vol. 22/1, 1981, pages 15 to 17. G. R. Spencer highlights some problems in using penetron screens in single anode cathode ray tubes. As is known different colours are produced using a dual primary colour penetron phosphor by varying the anode to screen voltages of the tube. One effect illustrated in broken lines in FIG. 3 of the Spencer article is that the spot size and thus the line width changes over the range of voltages that can be used. Accordingly the electron beam has to be refocussed if the spot size is to be maintained constant. Another problem with varying the anode to screen voltages is that in order to maintain a substantially constant picture size the deflection current has to be varied with screen current. G. R. Spencer proposes reducing the effects of these problems by separating the anode of the electron gun and the transparent electrode on the phosphor screen into two independent electrodes. However this dual electrode arrangement produces an increase in line width with increasing beam current and requires an increase in deflection current for increases in screen voltage.
One proposal for separating the scanning of an electron beam from the light and colour generation in a display tube employing a penetron screen is disclosed in British Patent Specification No. 1,402,547. This patent specification discloses a single beam display tube comprising a channel plate electron multiplier which comprises a stack of apertured dynodes the holes in which are aligned to form channels. A low energy electron beam is scanned across the input face of the electron multiplier. The electron multiplier produces a current multiplied electron beam which is used for light and colour generation. In Specification 1,402,547 a continuous two-layer red-green penetron phosphor layer is provided on the faceplate or other optically transparent carrier substrate disposed between the output surface of the electron multiplier and the faceplate. Additionally a blue light emitting phosphor is provided on a first colour selection electrode carried by the output surface of the electron multiplier and a second colour selection electrode is provided between the green penetron phosphor and the faceplate or its supporting substrate, the red penetron phosphor being closer to the electron multiplier than the green one. In operation, by varying the field set up between the first and second colour selection electrodes a selected one of the different phosphors can be activated. In the case of the blue phosphor not only must the electron beam emerging from the channel multiplier be turned through 180.degree. but also the light produced must be visible through the penetron screen. It is customary to provide an aluminum layer which is optically reflecting on the back of phosphor screens to increase the light output and sometimes also a carbon layer to reduce the effects of back-scattered secondary electrons from the phosphor screen, under such circumstances it is unlikely that the blue light will be visible therethrough.
Another approach to producing coloured images from a display tube including a channel plate electron multiplier is disclosed in British Patent Specification Nos. 1,446,774 and 1,452,554. This approach is based on the realisation that the electron beam emerging from a channel plate electron multiplier is hollow, that is it lands as a ring rather than a solid dot. Hence if the phosphor screen is made up of repeating groups of concentric phosphor rings, one for each of the three primary colours, and the focusing of the beam exiting from the channel plate electron multiplier can be changed in fixed amounts so that the beam impinges on each ring in turn, then a colour image can be produced. The resolution of the image is determined by two factors, firstly the pitch and size of the apertures in the channel plate electron multiplier itself and secondly the ability to lay down repeating groups of phosphor rings at a pitch to complement that of the apertures in the channel plate multiplier. For normal television applications, the phosphor repeat pattern has a pitch of between 0.7 and 0.8 mm and it is possible to lay patterns of phosphors to complement this pitch. However, for high resolution displays, for example data displays wherein a pitch of the order of 0.25 mm is desirable, there are practical difficulties in "shrinking" both the three colour phosphor pattern and adequately well focussed hollow electron beams to fulfil this requirement.